linux-ext4 - Re: Extended file stat: Splitting file- and fs-specific info?

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Date:	Thu, 10 May 2012 10:14:11 +0100
From:	David Howells <dhowells@...hat.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	dhowells@...hat.com, adilger@...ger.ca, bfields@...ldses.org,
	smfrench@...il.com, ben@...adent.org.uk,
	Trond.Myklebust@...app.com, roland@...k.frob.com,
	linux-fsdevel@...r.kernel.org, linux-nfs@...r.kernel.org,
	linux-cifs@...r.kernel.org, samba-technical@...ts.samba.org,
	linux-ext4@...r.kernel.org, linux-api@...r.kernel.org,
	libc-alpha@...rceware.org
Subject: Re: Extended file stat: Splitting file- and fs-specific info?

Dave Chinner <david@...morbit.com> wrote:

> > Also, do Dave Chinner's ideas for indicating five I/O parameters want to be
> > 32-bit numbers?  Larger?  Smaller?  Can they be log2?
> 
> Definitely 32 bit, IMO, as it's not uncommon to see optimal IO sizes
> in the tens of megabytes on large, high bandwidth storage systems.
> As for being log2 - that's just making it more complex to use and
> making code ugly - we'd have to convert to log2 in kernel, then
> convert back in every single application....

ilog2() in the kernel uses the CPU's bit-scan instruction if it has one and
converting back is just a bitshift operator.

But let's go with 32-bit fields for the moment.  I presume we aren't worried
about a driver that wants to do a 4GB transfer...

David
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